1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a method for manufacturing the same. Especially, the present invention relates to a vertical-cavity surface-emitting semiconductor laser for long wavelengths (i.e., 1.3 to 1.55 xcexcm) to be used as an optical source for a system of optical communication, optical interconnection, optical data-processing, or the like, in the field of optical data-communication or optical data-processing, and also to a method for manufacturing the novel vertical-cavity surface-emitting semiconductor lasers for long wavelengths.
2. General Background
Significant recent progress in the development of vertical-cavity surface-emitting lasers (VCSEL""s) emitting at 1.3-1.55 xcexcm is quickly making these light sources a viable option as high-performance components for optical fiber networks. In addition to offering cost advantages through such features as on-wafer testing, VCSEL""s also have inherent advantages over edge-emitters such as scalability to two-dimensional arrays. Although many of the best results for these devices have resulted from the wafer-fusion or metamorphic growth of AlGaAs-based distributed Bragg reflector (DBR) mirrors with active regions, there is still considerable interest in the monolithic growth of long-wavelength VCSEL""s. Essentially, lattice-matched, highly reflective AsSb-based DBRs eliminate the need for the complicated mirror schemes.
The mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses an Alx1Ga1xe2x88x92x1As Alx2Ga1xe2x88x92x2As material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum content x1 and x2 in the layers, more particularly the aluminum content x1 ranges from 0% to 50% and the aluminum content of x2 ranges from 50% to 100%. In conventional devices, the number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
In summary, a VCSEL includes a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region.
Lattice-matched, highly reflective AsSb-based DBRs eliminate the need for the complicated mirror schemes as mentioned above. Unlike AlGaAs DBRs, which have evolved over several years with many band-engineering schemes to produce electrically-mature mirrors, the AlAsSb/AlGaAsSb DBRs used in these lasers were, unfortunately, more resistive than would be desired. Combined with the poor thermal characteristics of these materials, the resulting high voltage leads to a large temperature rise in the active region which thereby limits the operation in these lasers to pulsed current.
Accordingly, in one embodiment of the present invention, a vertical-cavity surface-emitting laser (VCSEL) having at least one heat spreading layer is used for eliminating high voltage, thereby, allowing room temperature, continuous-wave (CW) operation of the VCSEL. The at least one of the heat spreading layers reduce the VCSEL temperature by allowing the injected current and generated heat to bypass at least one the DBRs which have poor electrical and thermal conductivity.
Accordingly, in another embodiment of the present invention, a vertical-cavity surface-emitting laser (VCSEL) for decreasing threshold current density comprises: (i) a first reflecting surface, (ii) a second reflecting surface, (iii) an active region with a first surface and a second surface, (iv) a first heat spreading layer preferably doped with an n-type material between the first reflecting surface and the first surface of the active region, (v) a second heat spreading layer preferably doped with an n-type material between the second reflecting surface and the second surface of the active region, (vi) an aperture formed by selectively etching the active region to a predetermined ratio of the size of the active region to the size of a DBR. The heat spreading layers reduce the VCSEL temperature by allowing the injected current and generated heat to bypass the reflecting surfaces which have poor electrical and thermal conductivity.